Effects of stress, defects, and layer composition on the development of corrosion features on protected silver mirrors.

The long-term environmental durability of protected silver mirror coatings is influenced by numerous factors. Here, accelerated environmental exposure testing of model silver mirror coatings illuminated the effects of stress, defects, and layer composition on the extent and mechanisms of corrosion and degradation. Experiments to reduce stress in the highest-stress layers of the mirror coatings revealed that, while stress may affect the extent of corrosion, coating defects and the composition of the mirror layers have the largest influence on corrosion feature development and growth.

Development and growth of corrosion features on protected silver mirrors during accelerated environmental exposure.

Various layer schemes have been developed to protect thin film silver mirrors from tarnish and corrosion. However, the mechanisms by which these additional layers improve mirror durability are not fully understood. Accelerated environmental exposure testing of protected silver mirrors was used to investigate the effects of layer composition on the mechanisms of corrosion feature development and growth. Two model mirror coatings were analyzed in which the composition of the base layer below the silver and the adhesion layer above the silver were varied. Large circular corrosion features formed preferentially along the silver-chromium interfaces, regardless of where this interface was located within the layered structure of the mirror. The corrosion features originated at coating defects in the dielectric protection layer, and their growth was likely driven by the oxidation and chloridation of both silver and chromium at the Ag-Cr interfaces.

Correlation of long-duration exposure and accelerated testing of protected silver mirrors.

The stability of two types of protected silver mirrors was studied by long-term exposure in a clean-room laboratory and mixed-flowing-gas (MFG) accelerated environmental test with two different gas concentrations. The two types of mirrors behaved very differently when exposed to the clean-room air for six years. The mirrors subjected to a reduced Battelle Class II MFG test protocol for 10 days exhibited similar corrosion mechanisms to those exposed to the clean-room air. With further testing of silver mirrors in both field exposures and MFG test conditions, it is feasible that the MFG-accelerated test method can be developed to quantitatively assess the durability of protected silver mirrors in ambient applications.

Environmental durability of protected silver mirrors prepared by plasma beam sputtering.

Various overcoat layers have been developed to protect silver mirrors from tarnish and corrosion. However, the mechanisms by which these protective layers improve mirror durability are not fully understood. Mixed flowing gas exposure was used to investigate the corrosion behavior of plasma beam sputtered silver mirrors with different adhesion layer materials. A small amount of nickel in the adhesion layer had a significant impact on the silver-dielectric interface. Additionally, lateral transport of silver was found to be an important factor in the corrosion process. Better stability at all layer interfaces is suggested to improve mirror durability.

Control of stress in protected silver mirrors prepared by plasma beam sputtering.

Plasma beam sputtering was used to deposit dielectric-protected silver mirrors that exhibited excellent durability and controlled stress. The durability of the mirrors was strongly dependent upon the presence of a very thin chromium adhesion layer between the silver layer and the dielectric overcoat. The stress of the five-layer mirror was balanced by controlling the compressive stress in the top dielectric layer, offsetting the net tensile stress of the combination of layers below.

Corrosion characterization of durable silver coatings by electrochemical impedance spectroscopy and accelerated environmental testing.

Highly reflective front-surface silver mirrors are needed for many optical applications. While various protective dielectric coating schemes have been developed, the long-term durability of Ag mirrors is still of great concern in the optics community for a variety of applications under harsh environments. The corrosion protection behavior of a SiNx-coated silver-mirror coating scheme was tested with electrochemical impedance spectroscopy (EIS) and accelerated environmental testing, including humidity and salt fog tests. The EIS data obtained were fitted with different equivalent circuit models. The results suggested that the 100A thick SiNx coating produced by rf magnetron sputtering was porous and acted as a leaky capacitor on the Ag film, whereas the addition of a NiCrNx interlayer as thin as 3A between SiNx and Ag films resulted in a much denser SiNx coating with a low-frequency impedance value of 2 orders of magnitude higher than that without the interlayer. Humidity and salt fog testing of different silver coatings showed similar results. The 100A SiNx/3A-NiCrNx/Ag coating exhibited excellent corrosion resistance against the corrosive environments used in this study.